The present invention generally relates to loudspeaker systems for sound reproduction and more particularly to a two-way or multi-way loudspeaker system.
A loudspeaker "system" as contemplated by the invention is a captive system including amplifiers, equalizers, cross-over filters, and acoustic transducers, sometimes referred to as "drivers," mounted in a speaker enclosure. The acoustical performance of such a system will be determined by the response and performance characteristics of each of its components and how each component interacts with one another. To a system designer, the goal is to have the overall system reproduce sound as close to the original source as possible. To do this the designer must achieve good transient response, which in the frequency domain is equivalent to a relatively flat amplitude response and relatively linear phase response versus frequency across the audio frequency spectrum. The impulse response of a perfectly linear system, using linear system theory, is a delayed "delta" function which causes no distortion, but only a net delay. Further characteristics of an "ideal system" against which performance improvement can be measured are described below in the Summary of the Invention.
One limitation in loudspeaker system design centers on the physical limitations of the acoustical transducer. It is generally not possible to force a single driver to operate over the full audio frequency spectrum efficiently enough to provide high quality sound reproduction. Consequently, loudspeaker systems often employ two or more drivers of different sizes and constructions dedicated to reproducing different parts of the audio frequency spectrum. These systems are called "two-way" or "multi-way" systems depending on the number of drivers used. In a two-way loudspeaker system, the audio input signal is electronically divided by cross-over circuits into two frequency bands or channels, namely, a high frequency channel and a low frequency channel, and each channel is fed to a different driver of the system. The advantage of the two-way system is that each individual driver operates under a reduced bandwidth, allowing optimization of the driver parameters, including reduction of distortion, power handling capability, and polar pattern response. However, increased complexity make it difficult to design a two-way (or multi-way) speaker system which is a highly accurate reproducer of sound. While good transient response in a one-way speaker system generally has not heretofore been obtained, the transient response of a conventional two-way or multi-way system is inherently worse by comparison. This problem principally has to do with the fact that the sound is not being emitted from one source or superimposed sources, but rather from two or more sources separated by finite distances.
Sources of distortion within the loudspeaker system impose further limitations on the designer's ability to achieve the goal of accurate sound reproduction. Distortion, measured by comparing the system's input and the output, arises from the presence of nonlinearities in the system's phase versus frequency response and/or from the system's amplitude versus frequency response to the extent it is not flat. Viewed in the time domain, distortion causes a degradation of the system's transient response, and hence the system's ability to reproduce an audio signal with high fidelity. A major source of distortion, and principally phase distortion, in a two or multi-way loudspeaker system is introduced by the presence of the cross-over circuits. Distortion is also introduced by the loudspeaker drivers themselves, and by other sources within the loudspeaker system.
Heretofore, one of the principal methods of optimizing the response characteristics of a two-way loudspeaker system has been to optimize the cross-over circuit to improve amplitude, phase, and polar response. Cross-over filters are designed using theoretical models which assume ideal drivers that exhibit flat amplitude and linear phase response and ideal acoustic environments (cabinet enclosures). Theoretical modeling also often assumes that the two sources of sound from the two drivers sum only in terms of magnitude and phase, ignoring the sound's direction of propagation or vector characteristics. Recently more sophisticated models have been described which take into account driver amplitude variations and optimize the cross-over design for a unique driver response. In all cases, the approach has been to design cross-over circuits which exhibit minimum amplitude and phase variation versus frequency over a specific polar pattern, and to do so from theoretical models.
The problem with such theoretic modeling is that individual drivers, and loudspeaker systems in general, are far from ideal, therefore the theoretical models make poor predictors of actual response. It is not uncommon for loudspeaker systems to exhibit more than 20 dB of amplitude variation across the audio spectrum on the radiation axis. The off axis errors are more than this, and because drivers are inherently band pass transducers, they have an associated phase shift which is usually non-linear as a function of frequency. When the drivers are combined with a cross-over circuit which has its non-linear phase shift in the cross-over frequency range, the non-linear phase distortion is compounded. The harmonic distortion of the drivers are usually quite high, often in the order of several percent. Drivers are also quite inefficient as acoustical transducers, often having less than a ten percent efficiency in terms of conversion of electrical input power to acoustical output power. These and other problems produce substantial errors in theoretical models and have limited past efforts to optimize the overall response characteristics of two-way and multi-way loudspeaker systems.
The present invention provides a correction circuit and method for improving the transient response of a two-way or multi-way loudspeaker system by correcting many of the above-mentioned sources of distortion which are not addressed or accounted for in conventional optimization schemes. The invention provides a circuit and method which improves the amplitude, phase and polar responses of a two-way (or multi-way) loudspeaker system in improving the system's transient response, not at just one point, but over an acceptable region in space.